Method of making electrically conductive thin epoxy bond

ABSTRACT

The present invention is a method and structure which produces extremely thin, electrically conductive epoxy bonds between two substrates. Copper microspheres, having an average diameter of about 2 microns are bound in an epoxy layer which bonds two substrates together. The microspheres make electrical contact between the substrates while providing intersphere gaps which are filled with the epoxy which actually bonds the substrates together.

BACKGROUND OF THE INVENTION

The present invention relates to a method of bonding two conductivematerials together with an epoxy and obtaining high quality electricalbonds therebetween.

In the manufacure of electronic devices, such as ultrasound transducers,it is often necessary to provide a means for electrically andmechanically attaching materials together, other than by soldering. Inparticular, in manufacturing ultrasound transducers, it is veryimportant that very thin bonds be made. Heretofore, the use of so-called"conductive epoxies" has not been found to be suitable, because"conductive epoxies"do not provide adequate bond strenth, and thematerial in conductive epoxies is extremely non-uniform, resulting inuneven bonds which are not suitable for ultrasound transducers, as themechanical properties of the bond affect the vibrational modes of thetransducers.

Accordingly, it would be highly desirable to provide a bonding materialwhich provides a thin, electrically conductive epoxy bond.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method of providing a thin,electrically conductive bond between two substrates is presented. Themethod is comprised of bonding the substrates together using an epoxy towhich electrically conductive microspheres have been added. In differentembodiments of the invention, different methods of adding themicrospheres to the epoxy are presented. The invention also comprisesthe composite material made in accordance with the inventive method.

BRIEF DESCRIPTION OF THE DRAWING

In the Drawing:

FIG. 1 illustrates a first embodiment of the present invention; and

FIG. 2 illustrates a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a first embodiment 10 of the present invention isshown. In this embodiment 10, a pair of substrates 12, 14 are shown. Thesubstrate 14 is comprised of a body of piezoelectric material 20 towhich a conductive layer 22 has been applied. The substrate 12 iscomprised of an electrically conductive body 16 to which a conductivelayer 18 has been applied. In the preferred embodiment of the invention,the electrically conductive layers 18, 22 are comprised of chromium,which is sputtered onto the bodies 16, 20. The substrates 12, 14 arejoined together by an electrically conductive epoxy bond 24 made inaccordance with the present invention. As shown, the bond 24 iscomprised of a series of electrically conductive microspheres 26(herinafter generally called "spheres") which electrically connect thesubstrates 12, 14 together. The spheres 24 are bound in a non-conductiveepoxy 28 which bonds the substrates 12, 14 together. Accordingly, thebond 24 is an epoxy bond formed by the epoxy 28 with the individualconductive spheres 26 bridging the thickness of the bond 24 to makeelectrical conduction between the two substrates 12, 14.

In accordance with the present invention, the conductive spheres 26 arepreferably comprised of a fine copper powder having an average diameterof approximately 2 microns. Such powder is of the type obtained fromCerac, P.O. Box 1178, Milwaukee, Wis. 53201. In the first embodiment ofthe invention 10, the copper powder is comprised of atomized copperspheres 26 which are bound in the epoxy base 28, which is preferablycomprised of Shell brand EPON 815 low viscosity epoxy using a Versamid140 curing agent.

In the first embodiment of the invention 10, the atomized copper spheres26 are applied to the surface 22 of the substrate 14. In order to insurethat the bond 24 is extremely thin, the surfaces 18, 22 of thesubstrates 12, 14 are highly polished prior to epoxying them. In theembodiment 10, the spheres 26 are sprayed from a suspension onto thesubstrate 14 or 2% by weight of the copper spheres 26 are mixed into theepoxy material 28 prior to laminating the two substrates 12, 14.

An evaluation of methods used to produce conductive bonds was conducted.In the test, glass slides were sputtered with a thin conductive twolayer film of nickel and chrome. First, nickel was sputtered directlyonto the glass for adhesion. Then, chrome was sputtered onto the nickelto provide a conductive layer to which a bond could be made. In a firsttest, a conductive epoxy called Epotek H-20E, a silver epoxy was used tojoin two prepared slides together. In a second test, copper microsphereswere sprayed on one slide prior to bonding in the manner of the presentinvention, and in a third test, 2% by weight of copper microspheres wasmixed directly into the epoxy. The slides were arranged in a row side byside and a large cylindrical pressure applicator spanned the slides atthe overlap joints.

The test was conducted at 60° Centigrade. It was found that theelectrical resistance of the bonds made by either the coppermicrospheres or by the Epotek were essentially identical. However, thebond strength in the case of the Epotek was only about 45 pounds persquare inch, while the bond strength provided by the present invention,i.e., epoxy mixed with microspheres, could not be measured due to glassfailure.

The thickness of bonded joints employing epoxy mixed with microsphereswas about 2.45 micrometers, whereas the Epotek joint was 12.25micrometers thick. A joint thickness of 12.25 micrometers is too largefor ultrasound transducer applications, at a frequency greater thanabout 7 MHz, as sound insertion losses causing severe bandwidthrestrictions will occur.

Microphotographs of the joints showed that when microspheres were usedthere was a single microsphere spanning the bond, whereas when theconductive epoxy (Epotek) was used, many particles are loaded into theepoxy in order to insure conductivity, so the bond is extremely thickand uneven, and it is not useful for high frequency ultrasoundapplications.

Referring now to FIG. 2, A second embodiment 30 of the present inventionis shown. In the embodiment 30, a first substrate 32 is electricallyconnected to a second substrate 34. The first substrate 32 is comprisedof a layer of piezoelectric material 36 having a sputtered chromiumlayer 38 applied thereon. Following the sputtering of layer 38 onto thelayer of piezoelectric material 36, the atomized spheres copper spheres40 are applied by any suitable means, such as by spraying them onto thesputtered layer 38. Then, an additional overcoat layer 42, preferablyalso of chromium, is sputtered onto the layer 38 with the spheres 40thereon. As shown, second sputtered layer 42 conforms to the shape ofthe spheres 40. Accordingly, when the epoxy 44 is applied over the layer42 and the second substrate 34 is pressed onto the epoxy 44, the epoxy44 fills the gaps surrounding the spheres 40 thereby providing adhesionbetween the layers 38, 40, while electrical conductivity occurs throughthe layers 38, 42 and the spheres 40. The purpose of the spheres 40 inthis embodiment 30 is to provide areas for the gaps for the epoxy 44 tofill while maintaining a very close spacing between the two substrates32, 34.

It has been found that by using the copper spheres an "invisible"acoustic bond between the substrates can be formed for use in ultrasoundtransducer manufacturing. Accordingly, virtually any matching layers maybe attached successfully to ceramic oscillators. Also, sheets oftransducer materials may be fabricated, resulting in consistent,reproducible individual transducers.

As will be obvious to those of ordinary skill in the art, the presentinvention provides a much better bond than the prior art which utilizedsurface roughness of one or both of the substrates or prior conductiveepoxy techniques.

I claim:
 1. A method of providing a thin, electrically conductive bondbetween two conductive substrates comprising the step of bonding thesubstrates together using a non-conductive epoxy to which uniformlysized, electrically conductive microspheres have been added, whereby thebond thus formed will be made by the non-conductive epoxy, the bondthickness will corrrespond to the diameter of said microspheres, andelectrical conductivity between said substrates will be provided by theconductive path formed by said microspheres.
 2. A method of electricallybonding two bodies together comprising the steps of:(a) sputtering aconductive layer onto each of said substrates; (b) applying uniformlysized electrically conductive microspheres to said sputtered layer onone of said substrates; and (c) bonding said substrates together using anon-conductive epoxy, such that said sputtered layers are in electricalcontact with said electrically conductive microspheres and saidnon-conductive epoxy bonds said substrates together, whereby the bondthus formed will by made by the non-conductive epoxy, the bond thicknesswill correspond to the diameter of said microspheres, and electricalconductivity between said substrates will be provided by the conductivepath formed by said microspheres.
 3. A method of electrically bondingtwo bodies together comprising the steps of:(a) sputtering a conductivelayer onto one of said substrates; (b) applying uniformly sizedelectrically conductive microspheres onto said sputtered layer; (c)sputtering a second conductive layer onto said first sputtered layerovercoating said microspheres and leaving gaps between said overcoatedmicrospheres; (d) filling said gaps with non-conductive epoxy; and (e)bonding said second body to said first body by applying said second bodyto said non-conductive epoxy, whereby electrical contact will occurthrough said sputtered layers and said microspheres.